The present invention relates broadly to a phase meter, and in particular to a digital phase meter apparatus.
Phase measurements are utilized to determine the relative times at which alternating currents and voltages in a circuit take on zero values. If two voltages v.sub.1 and v.sub.2 are zero at the same instant, they are in phase, with zero phase difference (or out of phase with 180.degree. difference). If one voltage v.sub.1 passes through zero 1/8 cycle before a second voltage v.sub.2, it leads by 360.degree./8 or 45.degree.. The common phase meter, a commercial device for determining the angle between current and voltage, can be used when its presence will not disturb the circuits under measurement. When phase angles to be measured are in high-impedance or low power circuits, this common device may be unsatisfactory and other measurement methods must be used.
The electronic phase-angle meter provides the angle (.pi.-.theta.) directly. One such prior art instrument converts the two voltage waves to square waves by repeated amplification and limiting. The zero crossings of the square waves are identical to the zero crossing of the original voltage waves. The two square waves are applied to the input of a circuit that will pass current only when both square waves are positive. In this case the greater the lag of one voltage, the smaller the overlap of the positive portions and the lower the average current. The current in this case is proporational to .theta..
This type of circuit has the theoretical limitation that each input voltage must be greater than a critical minimum value. In practice the critical value is determined by the noise on the amplifier input. If the voltage is too low, this noise causes a random zero-crossing shift and the results would be subject to this uncertainty.
A precision phase-angle meter for high frequency voltages uses a variable delay line, and its operation is based on the fact that the difference of two voltages of constant amplitude is a minimum when the two are in phase. One of the two voltages to be compared is connected to both inputs of a variable-delay line, which is then adjusted to give a minimum output. The two voltages to be compared are then connected to the two terminals and the delay line is readjusted to give a minimum output. The change in the delay-line setting indicates the time delay of one voltage relative to the other. When the frequency, is known, the time delay can be computed as angle of lag. If .DELTA.t is the change in the delay-line setting and the frequency is f hertz, the phase angle is given by 2.pi.f.DELTA.t radians or 360f.DELTA.t degrees.
Some prior art devices for electronically or digitally measuring phase difference are disclosed in the following U.S. Patents. For example, U.S. Pat. No 3,469,196 discloses a circuit for detecting the phase difference between two signals which includes means for squaring the two signals, and, first and second gates connected thereto for developing outputs whenever the two signals are simultaneously negative or positive respectively. U.S. Pat. No. 3,553,579 discloses a circuit for measuring the difference in phase between a reference signal and an unknown sinusoidal signal which includes a digital counter which provides a digital indication of the amount of phase displacement. A couple of references which are cited for general interest are U.S. Pat. Nos. 3,820,022 and 3,953,794 each of which discloses a circuit for measuring the phase difference between two signals.
The present invention, which utilizes a combination of digital elements to perform the function of measuring the phase difference between an input signal and a reference signal, is not disclosed by the prior art.